In conventional blood pressure measurement by the application of auscultation technique, the measuring is effected by using a manually operated air pump for pumping a pressure higher than systolic blood pressure into a blood pressure cuff which is usually fastened around the arm. Thereafter, the cuff pressure is reduced uniformly while listening to so-called Korotkoff sounds by means of a stethoscope. The cuff pressures corresponding to certain transition points in these sounds indicate systolic and diastolic blood pressures.
In manual blood pressure measuring, the measurer can arbitrarily control the rate of pressure reduction in such a manner that all factors having effect on measuring can be considered for setting thereby the rate of pressure reduction appropriate at a given moment. The pressure reduction can also be completely stopped and it is also possible to increase pressure with a hand pump during the measuring process.
An automatic blood pressure meter must be capable of controlling the rate of pressure reduction independently. This can be effected by measuring the cuff pressure and its reduction rate by means of a pressure transducer and by adjusting in accordance therewith a needle valve e.g. by means of a stepping motor. Since the same device must be capable of measuring with cuffs of varying sizes over a wide pressure range, a valve to be used in pressure reduction must be capable of operating over a wide range of volume flow. In order to maintain a constant rate of pressure reduction, it is necessary that a valve used for pressure reduction during the measuring be ajustable. This leads to sophisticated mechanical precision designs, whose industrial manufacturing is complicated and which are extremely sensitive to malfunctions caused by dust.